Staged Actuation Shear Sub for Use Downhole

ABSTRACT

A shear assembly used downhole to actuate a work string function includes a dampening subassembly to dampen the momentum of moving parts following shearing to avoid excessive acceleration and resulting jarring forces within the work string. A dual shear assembly is provided, in which the dampening subassembly prevents inertial force resulting from a first shear from exceeding the threshold for actuating the second, higher threshold, shear mechanism thereby allowing more precise control of the forces applied to the work string to prevent premature shearing of the second shear mechanism. In a specific embodiment, the dampening subassembly is a hydraulic dampening subassembly. The dual shear assembly may be used, for example, to first open a wash port above a tool lodged within the wellbore to loosen debris above the lodged tool, and then to shear the tool from the work string if necessary.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present invention claims the benefit of U.S. Provisional PatentApplication Ser. No. 61/012,457 entitled “Staged Actuation Shear Sub forUse Downhole” filed Dec. 9, 2007.

FIELD OF THE INVENTION

The present invention relates generally to tools for use within awellbore. More particularly, the present invention relates to a shearassembly for integration within a work string.

BACKGROUND OF THE INVENTION

During use of downhole equipment within a wellbore, sand and otherdebris may enter the wellbore and accumulate above the bottom holeassembly. From time to time, the tool may become lodged within thewellbore due to this accumulation of debris. Various means of attemptingto loosen lodged equipment are known, which generally include pulling(such as the controlled pulling taught by U.S. Pat. No. 7,249,633),vibrating, or reciprocating the bottom hole assembly within the wellboreuntil same is freed of debris. Unfortunately, these loosening methodsoften fail, requiring the bottom hole assembly to be sheared from thetubing string and fished out of the well. Such fishing operations areexpensive and time consuming, as an independent contractor musttransport specialized equipment to the site in order to fish thebottomhole assembly from the wellbore.

Once the tubing string has been sheared from the fishneck, the fishneckmay remain inaccessible due to further accumulation of debris duringshearing and removal of the tubing string. Accordingly, fishing/pullingequipment may incorporate a wash feature in order to loosen debris fromabout the fishing neck of a lodged downhole tool (see for example U.S.Pat. No. 3,020,957; U.S. Pat. No. 4,749,044; and U.S. Pat. No.5,887,925).

Similarly, U.S. Pat. No. 6,352,113 describes a submersible pump assemblyfor attachment within a tubing string, the pump assembly having a fluidtube extending from surface down to the bottom hole assembly to loosendebris from above the upper packer prior to pulling the bottom holeassembly from the wellbore.

To date, a shear-actuated wash mechanism has not been incorporatedwithin a work string above a bottom hole assembly for use in freeing thebottom hole assembly should it become lodged within the wellbore.Moreover, it has not to date been possible to include two tensionshear-actuated devices within a work string in close proximity, or withsimilar shear thresholds.

It is, therefore, desirable to provide a simple, staged device forwashing the area above a bottom hole assembly to free lodged equipment,and to shear the bottomhole assembly from the coiled tubing should thewash be ineffective in releasing the tool from the wellbore.

In addition to the desire for a tool able to clear debris from abovelodged downhole equipment, there is a more general need to provide astaged means of actuating downhole equipment or processes. Althoughtension shear devices are among the most simple means of tool actuation,the shearing of pins to actuate, open, or slideably operate an otherwisefixed element is generally avoided due to the unpredictable inertialforces that result from the application and release of tension withinthe coiled tubing. In other words, application of pulling force to thetubing string over a great vertical distance causes the coiled tubing tostretch. Thus, when the shearable element is released upon applicationof an appropriate threshold force at the shear location, the shearedelement will generally slide forcibly into a corresponding shoulder.This inertial force may be magnified by the simultaneous release oftension within the extended or stretched coiled tubing. Should a secondtension-shearable element be present in close proximity within the workstring, the inertial force of the moving parts immediately subsequent tothe first shearing event may be great enough to exceed the second shearthreshold unintentionally.

Due in part to the above-noted limitations associated with current sheardevices, typically only one tension shear-actuated function is usefulwithin a tubing string. As such, a shear sub is usually limited to useas a last resort mechanism by which the operator may separate the tubingstring from the equipment in an emergency or otherwise problematicsituation.

It is therefore further desirable to provide a shear assembly in whichrecoil force may be minimized or dampened, as this may facilitatedevelopment of a dual or multiple shear device for controlled, stagedactuation of downhole equipment or processes.

SUMMARY OF THE INVENTION

In accordance with a first aspect of the invention, there is provided ashear assembly for use downhole, the shear assembly comprising: amandrel for integration within a work string; a shear sleeve slideablewith respect to the mandrel to actuate a work string function; settingmeans for setting the shear sleeve in a fixed position with respect tothe mandrel, the setting means shearable by application of a thresholdforce to the work string, thereby actuating the work string function;and a dampening subassembly operatively associated with the mandrel andwith the shear sleeve for slowing the sliding momentum of the shearsleeve with respect to the mandrel following shearing of the settingmeans.

In accordance with a second aspect of the invention, there is provided adual shear assembly for use downhole, the dual shear assemblycomprising: a mandrel for integration within a work string; a firstshear sleeve slideable with respect to the mandrel to actuate a firstwork string function; first setting means for setting the first shearsleeve in a fixed position with respect to the mandrel, the firstsetting means shearable by application of a first threshold force to thework string, thereby actuating the first work string function; a secondshear sleeve slideable with respect to the mandrel to actuate a secondwork string function; second shearable setting means for setting thesecond shear sleeve in a fixed position with respect to the mandrel, thesecond setting means shearable by application of a second thresholdforce to the work string, thereby actuating the second work stringfunction; and a dampening subassembly operatively associated with themandrel and with the first shear sleeve for slowing the sliding momentumof the mandrel with respect to the first shear sleeve following shearingof the first setting means. It will be apparent that a multiple shearassembly, that is one with three or more shearing sub-assemblies, ispossible where n is the number of shearing functions and n-1 is thenumber of associated momentum-damping assemblies.

In an embodiment, the first work string function is actuated uponabutment of a mandrel surface with a shear sleeve surface, and whereinthe dampening subassembly slows the sliding momentum of the mandrel tolimit the force of said abutment.

In a suitable embodiment, the force of abutment is limited to a valueless than the second threshold force.

In an embodiment, the dampening subassembly is a hydraulic dampeningsubassembly. The hydraulic dampening subassembly may comprise one ormore annular pistons disposed within hydraulic chambers operativelyattached to the mandrel. The chambers may be filled with hydraulic fluidof suitable viscosity to provide the desired degree of dampening for aspecific application.

In an embodiment, the dampening subassembly reduces the abutment forceby at least 10,000 pounds force.

In an embodiment, the second threshold force exceeds the first thresholdforce by more than 15%.

In an embodiment in which the work string includes a frac packer, thefirst threshold force may be 40,000 pounds force or greater. Further,the second threshold force may be between 56,000 and 96,000 pounds.

In an embodiment, the first work string shear-initiated function isopening of a wash port within the work string.

In an embodiment, the second work string shear-initiated function isshearing of the mandrel from the work string, which may expose afishneck for use in recovery of the mandrel from the wellbore.

In accordance with a third aspect of the invention, there is provided amethod for returning a work string to surface when a downhole toolwithin the work string has become lodged within the wellbore, the methodcomprising the steps of: opening a wash port within the work stringabove the lodged tool; circulating fluid within the work string and thewellbore above the lodged tool to loosen debris from above the lodgedtool; and pulling the work string and tool to surface.

In an embodiment, the method further comprises the step of shearing afirst shearable setting means within the work string to open the washport.

In a further embodiment, the method further comprises the step ofshearing the lodged tool from the work string.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will now be described, by way ofexample only, with reference to the attached Figures, wherein:

FIG. 1 is a longitudinal cross sectional view of a dual shear assemblywith first and second sets of shear pins;

FIG. 2 is a longitudinal cross-sectional view of the assembly shown inFIG. 1, with the lower set of shear pins sheared and the mandrel shiftedto actuate wash ports;

FIG. 3 is a longitudinal cross sectional view of the assembly shown inFIGS. 1 and 2, with the upper set of shear pins sheared and the uppershear collar separated from the mandrel to reveal the mandrel fishneck.

DETAILED DESCRIPTION

Generally, the present invention provides a downhole shear assembly foractuating a work string tool or process. Specifically, the deviceincorporates dampening means for minimizing post-shear inertial forcewithin the work string and related equipment. Minimizing the post-shearinertial force prevents damage or premature actuation of sensitivecomponents within the work string, and permits more precise control offorces applied to work string tubing and equipment. This additionaldegree of precision in force application permits an additional shearpoint to be incorporated within the assembly for actuation of a secondprocess or function, the second shear mechanism having a higheractuation force threshold. In such dual shear devices, a first shearsleeve may be designated for initiation of a downhole process or toactuate a downhole tool, while the second shear sleeve (set to a higherforce threshold) may be assigned to initiate a second possiblyindependent, process. For example, the first shear collar may hold awash port closed, while the second, higher threshold shear collar maysecure a bottomhole assembly to the tubing string. With reference to theFigures, such an assembly is described below by way of example.

The following definitions are provided for the purpose of understandingspecific terms within the present specification.

“Work string” as mentioned herein refers to any means of conveyingequipment downhole, such as coiled tubing, jointed pipe, wireline,slickline, etc. that is lowered into a well to deliver downholeequipment (known as a bottom hole assembly) to an appropriate depth. Forclarity, work string refers to such lowering means with or withoutattached downhole equipment.

“Bottom hole assembly” refers to any downhole tool, equipment, orequipment assembly lowered downhole by or as part of a work string, forexample a fracturing bottom hole assembly or completion assembly,including packers, tubing anchors, or a variety of plugs.

Overview

In FIG. 1, a dual shear device 10 is shown for use within a work string.An upper shear sleeve 30 may be connected to a work string such as atubing string, while lower shear sleeve 40 may be connected to a bottomhole assembly or other downhole equipment. Both shear sleeves 30, 40 areslideably disposed over an inner mandrel 20. The basic operation of thedevice can be readily appreciated when viewed as three slideablyassociated functional portions (as referenced in FIG. 1):

A) inner mandrel 20, with fishneck 23, mandrel extension 21 and pistons22;

B) upper shear sleeve 30, which telescopes over fishneck 23 of the innermandrel 20, and is temporarily held in fixed position against the innermandrel 20 by upper shear pins 31; and

C) lower shear sleeve 40, with connected hydraulic chamber subs 41 andhydraulic cylinder cap 42, which together slide back and forth a givendistance over the remainder of the inner mandrel 20. As shown in FIG. 1,lower shear pins 43 extend through the lower shear sleeve 40 and mandrelextension 21 to fix the position of the lower shear sleeve and relatedcomponents with respect to the inner mandrel 20.

Notably, in this position the pistons 22 are nearest a first end (thelower end) of hydraulic chambers 44; the hydraulic cylinder cap 42 abutsthe lower facing surface of the upper shear sleeve 30; and mandrelextension 21 closes wash ports 45 within the lower shear sleeve. Itshould also be noted that in the present example, the lower shear pins43 are fewer in number than the upper shear pins 31, and are intended toshear at a lower force threshold than upper shear pins 31.

Application of a first threshold pulling force to the work string fromsurface will cause lower shear pins 43 to break, releasing the mandreland related components from the aforementioned fixed position.

However, the available sliding distance of the mandrel within the shearsleeve is limited by contact of the abutment portion 21 a of the mandrelextension 21 with the facing edge 41 a of the lowermost hydraulicchamber sub 41, as shown in FIG. 2. Such contact could potentially causea great jarring force within the work string as these componentsaccelerate towards each other following shear of the lower shear pins43. The resulting force of abutment would, in some circumstances,prematurely shear the upper shear pins 31. A hydraulic dampeningsubassembly C, particularly element 22, 42 and 44 is therefore presentto reduce this acceleration and resulting inertial force, limiting theforce of abutment to an acceptable amount (less than the threshold toshear the upper shear pins 31). Thus, the first and second shearmechanisms may be in close proximity along the work string, and thedifferential between the two shear thresholds can be minimized such thatboth thresholds are greater than the normal downhole operating range,while still being within the range of available pulling force fromsurface.

Lower shear pins 43 may therefore be sheared when necessary by theoperator to actuate a first work string function or process. In thepresently described embodiment, this first work string function is theopening of wash ports 45, as shown in FIG. 2.

Application of a further and higher threshold pulling force to the workstring will cause upper shear pins 31 to be broken, actuating a secondwork string function or process. In the presently described embodiment,this second work string function is the separation of the downholeequipment from the work string to expose the fishneck 23 of the innermandrel 20, as shown in FIG. 3.

Inner Mandrel and Related Components

The inner mandrel 20 shown in the Figures is of the type typically usedin the field. That is, the inner mandrel is generally a steel cylinderwith a fishneck 23 formed at one end to facilitate fishing of thedownhole equipment from the wellbore should this become necessary.Recesses are formed within the mandrel at appropriate locations toreceive the shear pins.

A mandrel extension 21 is threaded to the opposing end of the innermandrel. An abutment portion 21 a of the mandrel extension is of greaterouter diameter so as to slide within a corresponding mandrel guide 49formed within the lower shear sleeve. The mandrel guide 49 and abutmentportion 21 a therefore maintain the inner mandrel 20 and relatedcomponents (A) in association with the lower shear sleeve and relatedcomponents (C) even when lower shear pins 43 have been broken.Accordingly, should the upper shear sleeve 30 become disengaged from theinner mandrel 20 by shearing of the upper shear pins 31, the downholeequipment attached to the lower shear sleeve 40 may be removed fromdownhole by pulling the fishneck 23 to surface using known retrievalequipment and methods.

The length of the mandrel extension is appropriate to cover wash ports45 within the lower shear sleeve when the abutment portion 21 a is atthe lowermost end of the mandrel guide 49, and to reveal wash ports 45when the abutment portion 21 a is at the uppermost end of the mandrelguide 49.

With reference to the embodiment shown in the Figures, three annularpistons 22 may be fastened to the inner mandrel 20 at spaced apartlocations along its length.

The mandrel and mandrel extension include recesses for alignment withcorresponding apertures within the upper and lower shear sleeves,respectively. Once aligned, shear pins 31, 43, may be inserted therewithin to fix the shear assembly to both mandrel and mandrel extensionin appropriate configuration for use within a work string.

Upper Shear Sleeve

The upper shear sleeve 30 is machined from a steel cylinder, whichtelescopes or slides onto and over the fishneck 23 of the inner mandrel20 and includes shear pin apertures for alignment with correspondingapertures/recesses in the mandrel 20. Depth guide 32 assists in thisalignment, permitting the appropriate degree of telescoping over themandrel 20.

Lower Shear Sleeve

The lower shear sleeve 40 is machined from a steel cylinder so as to bethreaded at one end for attachment to a hydraulic chamber housing 41,while the opposing end is adapted to connect to downhole equipment, forexample a bottom hole assembly. The lower shear sleeve further providesa mandrel guide 49 to accommodate abutment portion 21 a of the mandrelextension 21 to limit sliding movement and maintain the attachment ofthese components.

Wash ports 45 are formed within the lower portion of the sleeve 30 asshown, and shear pin apertures are formed to align with correspondingrecesses/apertures in the mandrel extension. The lower shear sleevefurther includes a depth guide 46 to assist in this alignment.

Dampening Subassembly

The damping subassembly reduces the post-shear acceleration of themandrel due to release of tension within the coiled tubing. Inembodiments having more than one set of shear pins, the dampingsubassembly limits the amount of inertial force generated followingshearing a first set of shear pins, to a value that is less than theforce required to shear a second set of shear pins.

In a suitable embodiment, a hydraulic dampening subassembly provides theappropriate degree of dampening as above described. In the specificembodiment shown in the Figures, the subassembly includes threehydraulic chamber housings 44 filled with hydraulic fluid, and a piston22 for movement within each chamber 44 to control the acceleration ofthe mandrel with respect to the lower shear sleeve 40.

Hydraulic Chamber Housing—In the embodiment shown in the figures, thehydraulic chamber housings 41 are made as a cylinder of appropriateinner and outer diameter so as to provide a continuous outer diameter tothe shear assembly and to telescope over the inner mandrel 20. Thehydraulic chamber housings 41 are formed with threadable ends forconnection in series above the lower shear collar 40. Each housing 41provides one hydraulic chamber 44 about one piston 22 that haspreviously been fixed to the mandrel 20. A hydraulic chamber cap 42closes the uppermost hydraulic chamber. Once assembled, the hydraulicchamber housings 41 slide as a unit (C) with the lower shear sleeve 40over the pistons 22 and inner mandrel 20 (A).

The size of each hydraulic chamber 44 is sufficient to allow movement ofthe pistons there within as well as to house hydraulic fluid, which willbe displaced from one side of each piston to the other within thehousing upon shearing of the first set of shear pins 43. Seals areplaced at appropriate locations to seal the hydraulic chambers againstfluid leakage.

The number of hydraulic chambers included in the shear assembly isdependent upon the burst pressure of each chamber, and upon the shearthreshold. That is, when the threshold force is applied to the workstring at the shear assembly, the shear pins will break, causingpressure to build within each hydraulic cylinder as the pistons arestroked through the hydraulic fluid. An appropriate number of hydrauliccylinders should be present to avoid exceeding the burst pressure of thewalls of each individual hydraulic cylinder. Accordingly, theconfiguration of the piston and chamber, as well as the selection of anappropriate hydraulic fluid should be designed based on the shearthreshold and the desired degree of force dissipation, while theresulting burst pressure of each chamber will determine the number ofchambers required.

Pistons—The piston rings are attached to the mandrel, for example bythreads or using snap rings. Fluid flow is permitted past the piston byan aperture through the piston from its upper surface to its lowersurface. A seal is provided between the inner surface of the piston andthe mandrel on one side of the aperture, and between the outer surfaceof the piston and the inner surface of the hydraulic chamber housing onthe opposing side of the aperture.

In the embodiment shown, one aperture is present in each piston. Inother embodiments, it may be desirable to add further similar aperturesabout the circumference of the piston ring or to otherwise change thesize or configuration of the flowpath to adjust the available flow areapast the piston.

The aperture may include a check valve and/or choke in suitableembodiments for greater control over the flow within the hydraulicchamber.

The speed at which the pistons are able to move through the hydraulicfluid in response to a given degree of axial force will determine thedegree of dampening. When considering the degree of dampening, it shouldbe noted that upon application of a threshold axial force to the workstring, the sheared element will accelerate from the remainder of theassembly. While the tension in the coiled tubing will accelerate themandrel upwards, the lower shear sleeve 40, is fixed downhole. Mandrelextension 21 a forcibly contacts the lower edge 41 a of the lowermosthydraulic chamber housing 41, and the degree to which this multipliedforce will be dampened or absorbed by the dampening subassembly prior tothis forcible contact is determined by the equipment's ability to slowmovement of the mandrel, i.e. by slowing the movement of the pistonsthrough the hydraulic chambers. Accordingly, the available flow areacross section of the aperture of hydraulic fluid past the piston and thecomposition of the hydraulic fluid within the chambers will determinethe inertial force dissipated and that ultimately imparted to the workstring.

Hydraulic Fluid—A suitable hydraulic fluid is introduced into eachhydraulic cross section of each piston aperture (not shown) and eachchamber 44, having a viscosity appropriate to provide the desireddampening effect. Such considerations will be known to those skilled inthe art.

Parameters

In shallow gas well fracturing operations, for example, typical shearsubs for use in disengaging downhole equipment from the work string areset to shear at axial threshold forces less than 50,000 pounds force.However it is generally not practical to reduce this initial shearthreshold substantially, in this example as normal operating work stringtensions may be within the range of 30-40,000 pounds, and mayincidentally exceed the shear force when the string is in use, which maycause premature shearing. In different formations or in differentoperational situations these axial forces may be different. Inaccordance with the described embodiment, it is similarly advisable thatthreshold shear forces be set well above the range of normal operatingwork string tensions.

Conversely, the maximum force that can be exerted upon a work string islimited by the equipment at surface, depending also upon the depth anddeviation of the wellbore. For example, in a deviated well, drag of thework string against the deviated walls will significantly reduce thedegree of force that is ultimately applied at the bottomhole assembly,as the top-hole equipment and the work string each have force limits byvirtue of their design and materials. Reductions in force transmittalare also expected with increasing well depths. Accordingly, the shearthresholds should be set with these upper limiting factors in mind. Suchconsiderations may be readily made by calculations known to thoseskilled in the art.

In certain embodiments, it may be possible to incorporate three or moreindependently actuated tension shear mechanisms within one work string,with appropriate nested or sequential dampening subassemblies to preventpremature initiation of successive shear-actuable functions.

Operation

When operating a fracturing bottom hole assembly, the assembly maybecome lodged within the wellbore due to a build-up of debris above theupper packer. When the dual shear assembly shown in the Figures isintegrated within a work string above the bottom hole assembly, apulling force may be applied to the work string from surface to generatea first threshold axial force at the shear assembly. This will cause thelower set of shear pins 43 to break, and the mandrel 20 will slideupward with respect to the lower shear sleeve 40 until the mandrelextension 21 a contacts the lowermost hydraulic chamber housing 41. Theforce of this contact is dampened by the hydraulic dampening subassembly(including pistons 22 and hydraulic chamber housings 41), which slowsthe momentum of the mandrel with respect to the lower shear sleeve andbottom hole assembly prior to impact.

Once the sliding movement has been completed, wash ports 45 will beopen, and fluid may be pumped from surface through the work string tothe wash ports 45. Fluid exiting the wash ports 45 can then circulate inthe annulus between the work string and the wellbore above the washpoints and the bottom hole assembly. Should this circulation besufficient to dislodge the bottomhole assembly (which includes the upperpackers in the frac operation), the work string (including bottom holeassembly) may be pulled to surface. Conversely, should the circulationbe insufficient to release the bottomhole assembly, application offurther pulling force to the work string from surface will generate asecond above-threshold pulling force at the top or second shearassembly. Thus, the upper shear pins 31 will be broken and the innermandrel 20 will be released from the upper shear collar, exposing thefishneck 23 and releasing the work string from the bottom hole assembly(at and below the fishneck). The work string (without bottom holeassembly) is pulled to surface and the bottom hole assembly may beretrieved in the known manner by engaging and pulling on the fishneck23.

Other Embodiments

As shown in the Figures, the multiple shear sub system is provided as aunit. It is contemplated that various components of the system may beprovided as modules. For example, the mandrel may be segmented, allowingextension as necessary. Further, the mandrel and hydraulic chamber unitsmay be provided together as a module, with one or more units threadedtogether in series. In further embodiments, the function and orientationof the shear assemblies may be reversed or substituted; or downwardforce may be applied to the work string to shear the shear pins, ratherthan a pulling force from surface, with appropriate configurationchanges to the dampers and pins, etc.

While the above-described specific embodiments are provided toillustrate the invention, the present shear assembly and dampeningsystem may be applied to various fields and functions. Notably, it hasbeen stated above that the shear assembly may be used to actuate variousdownhole tools or processes. Such tools and processes may includeopening or closing of wash ports, tool release, releasing a ball orother actuating device downhole, opening or closing of frac ports,detonation of a perforating charge, applying a cleaning, fracturing,acidizing or other treatment fluid, or setting of packers, among otherpossibilities.

It is further noted that shear pins provide only one suitable settingand release means in accordance with an embodiment of the invention.However, any other suitable setting and release means may be used to setthe fixed position of the sliding components about a downhole mandrel,and to subsequently effect a dampened release and a second or subsequentrelease.

As the desired threshold longitudinal force for shearing any set ofshear pins depends on the specific downhole application andconfiguration, the degree of dampening required will also vary. Giventhe disclosure provided, it is assumed to be within the ability of aperson having ordinary skill in the art to adapt the dampening system ofthe presently described embodiment as necessary for use in a variety ofdownhole applications without further innovation or underexperimentation. Further, with respect to dampening subassemblies,alternate dampening subassemblies may be used and may be preferred incertain embodiments. For example, the recoil force may be absorbed by aseries of rubber or otherwise cushioning elements, foam or otherwisecompressible material, tapered deformable channels, detent mechanisms,pressurized air chambers, burst discs, springs, or different fluids,etc.

The above-described embodiments of the present invention are intended tobe examples only. Alterations, modifications and variations may beeffected to the particular embodiments by those of skill in the artwithout departing from the scope of the invention, which is defined bythe claims appended hereto.

1. A shear assembly for use downhole, the shear assembly comprising: a.a mandrel for integration within a work string; b. a shear sleeveslideable with respect to the mandrel to actuate a work string function;c. setting means for setting the shear sleeve in a fixed position withrespect to the mandrel, the setting means shearable by application of athreshold force to the work string, thereby actuating the work stringfunction; and d. a dampening subassembly operatively associated with themandrel and with the shear sleeve for slowing the sliding momentum ofthe shear sleeve with respect to the mandrel following shearing of thesetting means.
 2. A multiple shear assembly for use downhole, the shearassembly comprising: a. a mandrel for integration within a work string;b. a first shear sleeve slideable with respect to the mandrel to actuatea first work string function; c. first setting means for setting thefirst shear sleeve in a fixed position with respect to the mandrel, thefirst setting means shearable by application of a first threshold forceto the work string, thereby actuating the first work string function; d.a second shear sleeve slideable with respect to the mandrel to actuate asecond work string function; e. second shearable setting means forsetting the second shear sleeve in a fixed position with respect to themandrel, the second setting means shearable by application of a secondthreshold force to the work string, thereby actuating the second workstring function; and f. a dampening subassembly operatively associatedwith the mandrel and with the first shear sleeve for slowing the slidingmomentum of the mandrel with respect to the first shear sleeve followingshearing of the first setting means.
 3. The assembly as in claim 2wherein the first work string function is actuated upon abutment of amandrel surface with a shear sleeve surface, and wherein the dampeningsubassembly slows the sliding momentum of the mandrel to limit the forceof said abutment.
 4. The assembly as in claim 3 wherein the force ofabutment is limited to a value less than the second threshold force. 5.The assembly as in claim 2, wherein the dampening subassembly is ahydraulic dampening subassembly.
 6. The assembly as in claim 5, whereinthe hydraulic dampening subassembly comprises one or more annularpistons disposed within hydraulic chambers operatively attached to themandrel.
 7. The assembly as in claim 6 wherein the hydraulic chambersare filled with a hydraulic fluid of suitable viscosity to provide thedesired degree of dampening.
 8. The assembly as in claim 3 wherein thedampening subassembly reduces the abutment force by at least, 10,000pounds force.
 9. The assembly as in claim 2 wherein the second thresholdforce exceeds the first threshold force by more than 15%.
 10. Theassembly as in claim 2, wherein the work string includes a frac packer,and wherein the first threshold force is at least 40,000 pounds force.11. The assembly as in claim 10 wherein the second threshold force isless than 96000 pounds.
 12. The assembly as in claim 10 wherein thesecond threshold force is greater than 56000 pounds.
 13. The assembly asin claim 2 wherein the first work string function is opening of a washport within the work string.
 14. The assembly as in claim 2 wherein thesecond work string function is shearing of the mandrel from the workstring.
 15. The assembly as in claim 14, wherein said shearing of themandrel from the work string exposes a fishneck for use in recovery ofthe mandrel from the wellbore.
 16. A method for returning a work stringto surface when a downhole tool within the work string has become lodgedwithin the wellbore, the method comprising the steps of: a. opening awash port within the work string above the lodged tool; b. circulatingfluid within the work string and the wellbore above the lodged tool toloosen debris from above the lodged tool; and c. pulling the work stringand tool to surface.
 17. The method as in claim 16, further comprisingthe step of shearing a first shearable setting means within the workstring to open the wash port.
 18. The method as in claim 16, furthercomprising the step of shearing the lodged tool from the work string.